It is considered that a factor of the wear in the friction system between two objects is a friction between the objects. Accordingly, it has been conventionally considered that the wear is absorbed by interposing the lubricating oil between the objects, for example, between the cylinder and the piston in the internal combustion engine, whereby the wear is suppressed. In fact, it is apparent from experience that the wear is reduced by interposing the lubricating oil. However, the wear can not be completely prevented by interposing the lubricating oil in the friction surface, but can be only suppressed. The question lied in the fact that how effectively the wear can be suppressed with respect to the used time, whereby it is possible to improve the reliability of the internal combustion engine.
As is well known, it is recommended by professional manufacturer companies to replace an entire amount of the lubricating oil or the like in the internal combustion engine every specific period use or specific time use or drain a part of the lubricating oil and resupply a fresh oil. This is because the system oil of this sort generates an oxidative polycondensation effect in use, a combustion product is mixed thereto, and a metal wear particle, a rust or the like is contained therein, the system oil of this sort generates various insoluble contents. Among them, a solid body generated in the lubricating oil is increased with age at a time of being used and causes to damage the friction surface.
For these reasons, the professional manufacturer companies define respective nature limit values for the unique lubricating oils (refer to Table 1). The nature limit values are defined until now, for example, such that n-pentane insoluble content corresponding to a hydrocarbon of methane series contained in the lubricating oil is 1.5 mass % in a two-cycle engine and 2.5 mass % in a four-cycle trunk piston engine on the basis of an analysis in accordance with a method A. However, in fact, it is considered the nature limit value defining an allowable limit of using the lubricating oil varies in accordance with the size of a dust particle contained in the lubricating oil, a surface finish roughness in the friction surface, a kind of the generated material generated in the lubricating oil, the size of the particle, or a content thereof, it is hard to define the nature limit value, a clear base supporting the allowable limit has not been known, and the content of insoluble content is experientially set to be equal to or less than about 2.5 mass %. In other words, the lubricating oil is normally required to be replaced when the content of the insoluble content contained in the lubricating oil becomes more than 2.5 mass % as matters stands, and it is believed that the lubricating oil can be used without replacing the oil as far as the content is equal to or less than 2.5 mass %.
Table 1
However, the inventors have found that the conventional systems mentioned above do not suppress a deterioration of the internal combustion engine due to the wear on the friction surface between the cylinder and the piston to the minimum, rather they damage the surfaces of the cylinder and the piston corresponding to guts of the engine by using the lubricating oil at an impurity percentage content set as the allowable limit exceeding by far the optimum value of the nature of the lubricating oil, so that a performance of the engine is lowered, and a service life is significantly shortened.
In order to always keep the engine in an optimum state at a time of being used, it is necessary to take various factors into consideration. In particular, in order to keep the friction surface between the cylinder and the piston, between a bore and a shaft, or the like in an optimum state, the kind of the insoluble content such as the pentane or the benzene contained in the lubricating oil to be interposed in the friction surface as mentioned above and a corrosion content, the size of the particle, the content thereof, a fuel supply amount, an engine speed, a diameter and a size of the cylinder, a surface roughness of the friction surface, and the like are concerned. As a result of devoting themselves day and night to research over a long period, the inventors have found that the various factors affecting the nature of the engine can be represented by the percentage content of the insoluble content with respect to the lubricating oil interposed in the contact surface, and in the case that the percentage content of the insoluble content adopts a value within the specific value range corresponding to the engine speed or the maximum rotation speed, the wear on the friction surface is suppressed to the minimum, and reaches a completion of the present invention.